With recent intensification of safety regulations for passengers in a vehicle, precipitation hardening type high strength steel sheets have been widely applied to various structural components such as seat rails, pillars, etc. of a vehicle body for an improvement in impact resistance of the vehicle body. Since the precipitation hardening type high strength steel sheet is designed to absorb energy upon collision of the vehicle, it has a characteristic of a high ratio of yield strength versus tensile strength, that is, a high yield ratio (YS/TS).
Typical methods of strengthening steel can be summarized into solid-solution strengthening, grain-refinement strengthening, transformation hardening, and precipitation hardening. Among these methods, it is very difficult for the solid-solution strengthening and grain-refinement strengthening to produce high strength steel having a yield strength of 490 MPa or more with reference to the tensile strength thereof.
Furthermore, since not only does the transformation hardening require a great amount of alloying elements for insurance of strength and formation of transformation microstructure, but also the transformation hardening type steel has substructure of bainite or martensite, it is difficult to assure an excellent yield ratio. Thus, the trans-formation hardening is not appropriate for the components requiring the impact resistance in preparation for collision of the vehicle.
On the other hand, for the precipitation hardening, carbide and nitride formation elements such as Cu, Nb, Ti, V and the like are added to improve the strength via precipitation hardening and grain-refining effects. Thus, the precipitation hardening has a merit in that it can easily achieve high strength of the steel with low manufacturing costs. The precipitation hardening is carried out in such a way of performing solution treatment at high temperatures, followed by cooling the steel to form many fine precipitates, thereby strengthening the steel by virtue of a stress field around the precipitates.
Examples of precipitation hardening type high strength steel are disclosed in Japanese Patent Laid-open Nos. (Sho) 56-84422, (Hei) 4-221015, (Hei) 3-140412, and (Hei) 11-241119.
Techniques disclosed in Japanese Patent Laid-open Nos. (Sho) 56-84422 and (Hei) 4-221015 are to produce a precipitation hardening type high strength steel, which comprises a low content of carbon as a primary component and one or more components selected from Ti, Nb, V and the like as a secondary component, through control of hot rolling and coiling temperatures. The techniques of the disclosures are very effective to improve the strength of the steel by formation of ultra-fine pre-cipitates due to a very low coiling temperature. However, not only do these techniques have difficulty in ensuring the yield strength of 750 MPa or more, but also often suffer from overload during cold rolling due to an increase in residual stress around the precipitates.
Techniques disclosed in Japanese Patent Laid-open Nos. (Hei) 3-140412 and (Hei) 11-241119 are to produce a precipitation hardening type high strength steel by use of Cu precipitates. The techniques of these disclosures are advantageous to ensure the strength of the steel by use of the Cu precipitates, but suffer from alloying defects of a plated steel sheet due to the Cu precipitates and insufficient weldability, which make it difficult to apply the steel in practice.
Ultra strength steel having the yield strength of 750 MPa or more can be produced through particular methods such as addition of great amounts of alloying elements, recovery annealing, transformation control, etc.
One example of such methods is disclosed in Korean Patent Application No. 2004-111413, which produces a high strength steel sheet having the yield strength of 750 MPa or more through recovery annealing by use of steel which comprises, by weight %, C: 0.08˜0.12%, Mn: 1.8˜2.2%, and suitably controlled amounts of Nb and Mo.
However, the method disclosed in Korean Patent Application No. 2004-111413 has a relatively high content of Mn, and suffers from severe Mn-oxide enrichment on the surface of the steel sheet. As a result, since the steel sheet is likely to be deteriorated in quality due to damage of dies during automotive die machining, this method is not suitable for application to the steel sheet for the vehicle.